The peak–end rule is a theory that describes how humans hedonically evaluate past experiences. This heuristic process leads people to judge an experience by its most intense point and its end, as opposed to the total sum or average of every moment of the experience. It occurs regardless of whether a ‘peak’ is pleasant or unpleasant, and regardless of the duration of the experience.
The peak–end rule is an elaboration on the snapshot model of remembered utility proposed by Barbara Fredrickson and Daniel Kahneman. This model dictates that an event is not judged by the entirety of an experience, but by prototypical moments - “snapshots”- as a result of the representativeness heuristic. "Snapshots'" remembered value dominates the actual value of an experience. Thus, Fredrickson and Kahneman were able to theorize that these "snapshots" are actually the average of the most affectively intense moment of an experience and the feeling experienced at the end. The effects of the duration of an experience upon retrospective evaluation are extremely slight. Frederickson and Kahneman labeled this phenomenon duration neglect . The Peak-End rule is applicable only when an experience has definite beginning and end periods.
Research and examples
A 1993 study titled “When more pain is preferred to less: Adding a better end,” by Kahneman, Fredrickson, Charles Schreiber, and Donald Redelmeier provided groundbreaking evidence for the Peak-End rule. Participants were subjected to two different versions of a single unpleasant experience. The first trial had subjects submerge a hand in 14°C water for 60 seconds. The second trial had subjects submerge the other hand in 14 °C water for 60 seconds, but then keep their hand submerged for an additional 30 seconds, during which the temperature was raised to 15 °C. Subjects were then offered the option of which trial to repeat. Against the law of temporal monotonicity, subjects were more willing to repeat the second trial, despite a prolonged exposure to uncomfortable temperatures. Kahneman et al. concluded that “subjects chose the long trial simply because they liked the memory of it better than the alternative (or disliked it less).” 
Similarly, a 1996 study by Kahneman and Redelmeier assessed patients’ appraisals of uncomfortable colonoscopy or lithotripsy procedures and correlated the remembered experience with real-time findings. They found that patients consistently evaluated the discomfort of the experience based on the intensity of pain at the worst (peak) and final (end) moments. This occurred regardless of length or variation in intensity of pain within the procedure.
A second study by Kahneman, Redelmeier, and Joel Katz, corroborated and expanded upon the discoveries made in the 1996 study. Colonoscopy patients were randomly divided into two groups. One underwent a colonoscopy procedure wherein the scope was left in for three extra minutes, but not moved, creating a sensation that was uncomfortable, but not painful. The other group underwent a typical colonoscopy procedure. Kahneman et al. found that, when asked to retrospectively evaluate their experiences, patients who underwent the longer procedure rated their experience as less unpleasant than patients who underwent the typical procedure. Moreover, the patients in the prolonged discomfort group were far more likely to return for subsequent procedures because a less painful end led them to evaluate the procedure more positively than those who faced a shorter procedure.
Since most consumer interactions have set beginnings and ends, they fit the peak–end model. As a consequence, negative occurrences in any consumer interaction can be counteracted by establishing a firmly positive peak and end. This can be accomplished through playing music customers enjoy, giving out free samples, or paying a clerk to hold the door for patrons as they leave. As Scott Stratten has suggested, “A really great salesperson who helps with an exchange can erase negative experiences along the way. The long wait in line and the bad music in the changing room are forgotten”, However, as research by Talya Miron-Shatz suggests, retrospective evaluations of day-long experiences do not appear to follow the peak–end rule, which brings the applicability of this rule to certain types of consumer–business interactions of approximately day-length duration – like hotel stays – into question.
In a 2008 study, 49 college students on vacation for an average of seven days sent daily text messages evaluating their happiness over the previous 24 hours. After their vacations, they were asked to recall their daily happiness and the overall experience of the trip. The duration of the vacation had no effect on the evaluations made after it, and participants were unable to recall their happiness levels day to day. Students did not remember individual moments and then average them in their evaluation of the vacation. Additionally, the average happiness experienced over the trip was much lower than the remembered happiness because the peak–end effect alters recollection.
Despite this, the Peak-End rule would tend to suggest that people should take shorter vacations, since all that will be remembered are the vacation’s most salient point and end, which, taken together, usually amount to very little time.
The Peak-End rule is particularly salient in regard to medical procedures, since it suggests that it is preferable to have longer procedures that include a period of decreased discomfort than to have shorter procedures. In particular, "The Peak-End rule suggests that the memory of a painful medical treatment is likely to be less aversive if relief from the pain is gradual than if relief is abrupt”. Furthermore, the quality of a remembered procedure can drastically influence medical futures. If people recall necessary but onerous procedures more positively, then they are more likely to return for repeat procedures later in life. However, factoring the effect of the peak–end rule upon evaluations of medical procedures is problematic, since adding a period of decreasing pain to a procedure is still added pain. Even though this certainly yields a better memory of the process, the patient still endures more pain than is strictly necessary. Doctors and patients are forced to confront the choice between objectively less painful forms of treatment and forms of treatment that will be remembered more favorably. Kahneman claims, “it is safe to assume that few patients will agree to expose themselves for the sole purpose of improving a future memory”.
Desirability of life
The peak–end rule suggests that the quality of a life is heavily contingent upon the intensity of that life in general and at its end, not on the quality of the life as a whole. Therefore, the impact of the peak–end rule fundamentally calls into question the desirability of long lives. Since long lives almost inevitably conclude in a decline, a long life might not be the most desirable outcome. The fact that people prefer good lives that end abruptly before they can decline - known as the James Dean Effect - implies that endeavors to indefinitely prolong life are at best unnecessary and at worst acutely detrimental. This suggests that facilities that specifically facilitate extreme longevity, such as old age homes, can unknowingly serve to reduce perceived quality of life. It should be noted that the peak-end rule may not apply to end-of-life decision making since the rule is a heuristic process for judging past experiences, which will not occur after death. Therefore, it may be inappropriate to apply the peak–end principle to a life in its entirety.
Critiques of the peak–end rule typically derive from its conflation of a complex mental evaluation into a simplistic framework. For example, the peak–end rule is not an outstandingly good predictor of remembered experiential value. First impressions, unusual experiences, or memorable thoughts may be more salient than positive or negative experiences. The extreme effect of peaks fades more rapidly over time causing peaks to be recalled less positively and troughs recalled less negatively over time. Episodic memory endures for only a few weeks; at some point mental accounting shifts over to semantic memory, leading to potential over-valuation of the ‘end’ and diminished weighting of the peak. Additionally, memories that are available for evaluation may change due to the fading affect associated with memory or differing goals in recall. Goal orientation or initial expectations can also affect the weighting of a peak or an end, causing an end to be over-weighted as the culmination of a goal. Finally, Ariely and Carmon have theorized that evaluations of past events are affected by feelings at the time of evaluation.
- Fredrickson, B. L., & Kahneman, D. (1993). Duration neglect in retrospective evaluations of affective episodes. Journal of Personality and Social Psychology, 65(1), 45-55. doi:10.1037//0022-35188.8.131.52
- Kahneman, D. & Tversky A. (1999). Evaluation by Moments: Past and Future. In Choices, Values and Frames (pp. 2-23). New York, NY: Cambridge University Press. doi: 
- Kahneman, D., Fredrickson, B.L., Schreiber, C.A., & Redelmeier, D.A. (1993). When more pain is preferred to less:. Adding a better end. Psychological Science, 4(6), 404. doi:10.1111/j.1467-9280.1993.tb00589.x
- Redelmeier, D.A., & Kahneman, D. (1996). Patients' memories of painful medical treatments: Real-time and retrospective evaluations of two minimally invasive procedures. Pain, 66(1), 3-8. doi:10.1016/0304-3959(96)02994-6
- Redelmeier, D., Katz, J., & Kahneman, D. (2003). Memories of colonoscopy: A randomized trial. Pain, 104(1-2), 187-194. doi:10.1016/S0304-3959(03)00003-4
- Miron-Shatz, T (2009). "Evaluating multiepisode events: Boundary conditions for the peak-end rule". Emotion 9 (2): 206–13. doi:10.1037/a0015295. PMID 19348533.
- Kahneman, D. (1999). Objective Happiness. In Kahneman, D., Diener, E. and Schwarz, N. (eds.). Well-Being: The Foundations of Hedonic Psychology. New York: Russel Sage. pp. 3–25.
- Open-source pre-publication manuscript: Kahneman D, "Experienced utility and objective happiness: a moment-based approach", in Choices, Values and Frames, D. Kahneman and A. Tversky (eds), New York: Cambridge University Press.